1. Field of the Invention
The invention relates to a developer for irradiated, radiation-sensitive recording materials which contains water and at least one compound which is alkaline in aqueous solution. The developer is particularly suitable for developing radiation-sensitive recording materials from which offset printing plates are produced. The invention is also directed to methods of making and using the developer.
2. Description of Related Art
Aqueous alkaline developers for recording materials should generally meet a number of requirements. For example, they should have high productivity, i.e., the developing capacity should decrease as little as possible after prolonged use and hence increased contamination by detached components of the layer. The so-called "tone limit" at which the developer is spent and the layer is removed only incompletely from the nonimage parts should be as high as possible. The developers should develop the recording materials as rapidly and completely as possible. This is particularly important in the mechanical production of the printing plates in processing lines. Owing to the short cycle times, a development time of substantially less than 30 seconds may be necessary.
For ecological and economic reasons, it is advantageous if the developer can be offered in the form of a liquid concentrate which the consumer himself dilutes with water. Although the hardness of the water differs from region to region, this should not influence the properties of the ready-to-use developer. Thus, it is desirable that the developer be substantially unaffected by water hardness.
During development, neither foam nor solid precipitates should form. The latter might contaminate the development units and hence give rise to a high level of maintenance or even settle on the developed recording material.
Moreover, the developer solution laden with layer residues should be compatible with the gumming solutions used in the processing line. Owing to the unavoidable entrainment of contaminated developer solution by printing plates passing through, layer residues may accumulate after a relatively short time in the downstream storage container of the gumming solution. In the most unfavorable case, this gives rise to solid precipitates which, after a phase of concentration, are redeposited on the printing surface of the developed printing plate via the roller rail system of the gumming station and can thereby render the plate unusable.
EP-A 0 323 836 describes aqueous alkaline developers for positive- and negative-working recording materials. In addition to an alkaline composition, they contain a thickener which adjusts their viscosity to 5 to 10,000 cps. If necessary, the developers may also contain organic solvents, such as benzyl alcohol or ethylene glycol monophenyl ether. Developers for positive-working recording material may furthermore contain surfactants, in particular anionic or amphoteric surfactants. The thickeners used are fine silicate powders or water-soluble polymers such as starch, gum arabic, sodium alginate, pectin, viscose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, polyvinyl methyl ether, or polyvinyl-pyrrolidones.
A very specific aqueous alkaline developer for negative-working recording materials for the production of offset printing plates is disclosed in EP-A 0 602 736. This developer permits particularly short development times and prevents excessive deposits in the processing units. Moreover, it is said to rapidly dissolve deposits already formed. This developer contains an organic solvent, an anionic surfactant, an alkali metal tetraborate, an alkali metal salt of an aliphatic monocarboxylic acid, an alkali metal salt of an aliphatic dicarboxylic acid, an alkali metal citrate, a poly(alkylene glycol), an alkali metal salt of a hydroxylated aromatic carboxylic or sulfonic acid, and an alkaline buffer system which ensures an alkaline pH.
A developer suitable for the production of negative-working printing plates is also the subject of U.S. Pat. No. 3,954,472. It contains ammonium sulfite and polyvinylpyrrolidone dissolved in water and an alcohol, in particular n-propanol. The polyvinylpyrrolidone acts as a gumming composition.
Polyvinylpyrrolidone or a copolymer having units of N-vinyl-pyrrolidone is also used in the aqueous alkaline developer according to DE-A 42 24 205. The developer furthermore contains an alkaline agent such as NaOH, KOH, Na.sub.2 CO.sub.3, K.sub.2 CO.sub.3, sodium silicate, potassium silicate, trisodium phosphate, or tripotassium phosphate. If necessary, a surfactant of the alkali metal sulfate or alkali metal sulfonate type, an antifoam and an organic solvent, such as benzyl alcohol or ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether or ethylene glycol monophenyl ether, may also be added. It is envisaged for use in particular for the development of color proof materials, where it prevents the formation of stains in the nonimage parts.
EP-A 0 004 014 discloses a particularly productive developer for recording materials having exposed, negative-working layers of diazonium salt polycondensates and water-insoluble organic polymers. It contains an anionic wetting agent, polyvinyl alcohol, poly(N-vinyl-N-methylacetamide), benzyl alcohol, glyceryl triacetate and a buffer which ensures a pH in the range from 4 to 8. Since only high-boiling organic solvents are used in this developer, its composition remains virtually constant even on prolonged standing in open containers.
With the aqueous alkaline developer according to EP-A 0 180 122, it is possible to achieve particularly rapid and good development of exposed, negative-working photoresist layers. It also has a long life, can be used in automatic developing machines and prevents the formation of cakes and small cakes of detached layer components. In addition to water and an alkaline agent, it contains a complexing agent, an anionic surfactant, an alkanoic acid or a salt thereof, a buffer substance and an emulsifier, which is preferably poly(N-vinyl-N-methylacetamide), polyvinyl alcohol, dextrin, gum arabic, or a cellulose ether.
Although the stated polymeric emulsifiers result in a substantial improvement in the developer properties, they also have disadvantages. For example, they substantially affect the viscosity, i.e., they result in a relatively high viscosity even when they are contained in the developer only in small amounts. However, having as low an initial viscosity as possible and a moderate increase in viscosity with increasing contamination of the developer by detached layer components are important factors for minimizing the mechanical load on components of the processing line (for example, pumps and hose connection) on the one hand and, on the other hand, for counteracting premature sludge formation in the developing section of the processing machine, resulting in greater cleaning work when the developer is changed. With the known emulsifiers, it is always only a compromise that it achieved. Owing to the strongly viscosity-increasing effect of the known emulsifiers, their amount in the developer solution should often be limited.